Choppers in the form of rotary shutters have been used for locating the position of optical signals, such as the radiation emitted by an object, for the purpose of determining the coordinates representing the direction towards such an object. The radiation emitted by the object is projected as an image point on the chopper plane. The chopper comprises one or more clearly defined opaque portions which prevent the incident radiation from passing through the chopper. This means that when the chopper is rotating a plurality of pulses are produced by a photodetector which is positioned behind the chopper. The pulse signal emitted by the photodetector is evaluated with respect to the radial distance of the image point from the center of the chopper and also with respect to its angular position, which makes it possible to determine the direction towards the radiation source. One example of such a measuring device is described in Swedish patent application No. 76 13514-4, which corresponds to U.S. Pat. No. 4,178,505. The signals emitted by a remote object are often very small which means that a high degree of amplification of the signals is required. Due to the background noise such a high degree of amplification often makes the signal indecipherable to the signal processing means of the measuring device. There is a substantial risk that one or more signal pulses will be contaminated with the background noise with the result of an erroneous evaluation.